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Abstract

The temporal and spatial dynamics of soil water are closely interlinked with terrestrial
ecosystems functioning. The interaction between plant community properties
such as species composition and richness and soil water mirrors fundamental
ecological processes determining above‐ground–below‐ground feedbacks. Plant–
water relations and water stress have attracted considerable attention in biodiversity
experiments. Yet, although soil scientific research suggests an influence of
ecosystem productivity on soil hydraulic properties, temporal changes of the soil
water content and soil hydraulic properties remain largely understudied in biodiversity
experiments. Thus, insights on how plant diversity—productivity relationships
affect soil water are lacking.
2. Here, we determine which factors related to plant community composition (species
and functional group richness, presence of plant functional groups) and soil
(organic carbon concentration) affect soil water in a long‐term grassland biodiversity
experiment (The Jena Experiment).
3. Both plant species richness and the presence of particular functional groups affected
soil water content, while functional group richness played no role. The effect
of species richness changed from positive to negative and expanded to
deeper soil with time. Shortly after establishment, increased topsoil water content was related to higher leaf area index in species‐rich plots, which enhanced shading. In later years, higher species richness increased topsoil organic carbon,
likely improving soil aggregation. Improved aggregation, in turn, dried topsoils in
species‐rich plots due to faster drainage of rainwater. Functional groups affected
soil water distribution, likely due to plant traits affecting root water uptake depths,
shading, or water‐use efficiency. For instance, topsoils in plots containing grasses
were generally drier, while plots with legumes were moister.
4. Synthesis. Our decade‐long experiment reveals that the maturation of grasslands
changes the effects of plant richness from influencing soil water content through
shading effects to altering soil physical characteristics in addition to modification
of water uptake depth. Functional groups affected the soil water distribution by
characteristic shifts of root water uptake depth, but did not enhance exploitation
of the overall soil water storage. Our results reconcile previous seemingly contradictory
results on the relation between grassland species diversity and soil moisture
and highlight the role of vegetation composition for soil processes.